Pneumatic radial tire including a tread portion divided into four circumferential regions

ABSTRACT

A pneumatic radial tire comprising a carcass and a belt comprising belt plies composed of steel belt cords wherein at least two circumferential grooves are constructed in the tread part continuously in the direction of the tire&#39;s equator, and lateral grooves in some parts defined by dividing the tread part in the direction of the tire&#39;s axis into approximately three or four equal areas. The tire prevents one-side drifting of a car in driving, improves a straight-forward driving performance.

This application is a continuation of application Ser. No. 07/695,324filed on May 3, 1991, now abandoned, which is a divisional ofapplication Ser. No. 07/441,973 filed on Nov. 28, 1989, now abandoned.

BACKGROUND OF THE INVENTION

The present invention relates to a pneumatic radial tire, and moreparticularly to a pneumatic radial tire which reduces one-side driftingin driving and can be preferably used for passenger cars.

Radial tires which can improve steering stability, comfortable ridingperformance and so on are widely used. Tires which are superior instraight-forward driving performance by preventing one-side drifting ofa car in order to increase the driving safety of a car are beingdemanded. Conventionally, one-side drifting is considered to be causedby the so-called conicity in which the circumferential lengths of thebelt layer in the right and left sides in a tyre are diferent from eachother. Therefore, various methods have been taken in order to improvethe homogeneity in the right and left directions of the tire's axis.

On the other hand, due to the recent progress in tire measurementtechniques, as shown schematically in FIG. 9, a cornering force, i.e. alateral force F which is generated in the lateral direction Y of a tirewhen a tiny slip angle (α) is given in the running direction X of thetire and a self-aligning torque SAT which revolves in the direction ofthe slip angle (α) about the vertical axis Z that passes the center of atire can be measured at a high precision.

Such measurement results are shown using curve K in FIG. 10, by plottingthe self-aligning torque SAT on the abscissa axis and the lateral torqueF on the ordinate axis. In the curve K, the cases when the slip angle(α) in 0 deg., +0.1 deg. or -0.1 deg. are shown with a dot.

In such relation of self-aligning torque SAT and lateral force F, thelateral force F at the crossing point K1 of the curve K and the axis ofordinates, that is, the lateral force P when the self-aligning torqueSAT is not generated, is called a residue CF. It was found that theresidue CF is a tire characateristic which affects the one-side driftingof a car. In other words, a car is drifted to one side in the rightdirection when the residue CF is in the plus direction, i.e. the rightdirection. Thus, the one-side drifting characteristic of a car can beevaluated by the direction and the size of the residue CF. In order toprevent the one-side drifting of a car, it is required to reduce theresidue CF.

The residue CF is generated from the expansion and contraction of thebelt at the ground contact part. A shearing strain in a surface iscreated in the cross ply belt of a radial tire by the parallel movementof the cords due to the expansion and contraction. Thus, the threadrubber generates a steering torque by a shearing strain in a surfacegenerated together with the strain of the belt ply in the outermostlayer. It is considered that the lateral force F is created by thissteering torque. Thus, it was found that the residue CF is caused by abelt and depends on the cord quantity of the belt and the inclination ofthe belt cords.

The cord quantity is defined as N×S, which is the product (in mm²) ofthe total cross-sectional area of one belt cord S (sq.mm) and the numberof belt cords N laid in 10 cm in a right-angled direction to the beltcords. In other words, the cord quantity is the total cross-sectionalarea (mm²) of the belt cords N per 10 cm width of the belt ply. Therigidity of the belt can be reduced by reducing the cord quantity N×Sand enlarging the inclination angle of the cords to the direction of thetire's equator. It is known that this reduces the hooping effect of thebelt and then the residue CF, thereby controlling the one-side driftingof a car.

On the other hand, such reduction of the rigidity of the belt canimprove the comfortable riding performance at the same time, which is abasic item required for a car.

Additional experiments were conducted about the reduction of the residueCF in a tread part having a relatively low belt rigidity. As a result,it was found that good results could be obtained by reducing theinclination of the lateral grooves crossing the circumferential grooves,that is, constructing the lateral grooves closely in the direction ofthe tire's axis. However, such a tread pattern does not appeal tocustomers, because it lacks an aesthetic sense, and tends to decreasethe marketability of the product.

SUMMARY OF THE INVENTION

It is hence a primary object of the first and the second embodiments ofthe present invention to present a pneumatic radial tire which preventsone-side drifting of a car when driving, improves the straight forwarddriving performance and helps to improve the aesthetic sense.

However, by reducing the belt cord quantity and enlarging theinclination angle to the direction of the tire's equator, the hoopingeffect is reduced and the cornering force decreases, especially uponturing, thus impeding the steering stability.

It is hence a primary object of the third, fourth and fifth embodimentsof present invention to present a pneumatic radial tire which preventsone-side drifting of a car when driving, improves the straight forwarddriving performance and reduces the deterioration of steering stability.

In the first and second embodiments of the present invention asdescribed above, by setting the inclination angle of lateral grooves tothe direction of tire's axis at a small angle, the residue CF can bereduced in a tire. However, in order to increase the product appeal andthe marketability of a tire, its tread pattern must follow the aestheticsense of customers. But, a pattern of lateral grooves extending in thedirection of tire's axis is mainly perceived to lack powerfulness.Therefore, in order to increase the perceived powerfulness, theinclination should be larger, but on the other hand, a largerinclination accompanies an increase of the residue CF. Therefore, it isrequired to meet these contradictory requirements.

Consequently, in the first embodiment of the invention, an increase ofthe residue CF is prevented and an image of the pattern can be improvedby constructing approximately symmetrical lateral grooves in a V shapein at least one outside area and inside area in a tread part divided inthe direction of tire's axis approximately into four equal areas whichare left inside area CL, right inside area CR (combined and called aninside area C), left outside area SL and right outside area SR (combinedand called an outside area S), with a difference of inclination angles(|Θ1|-|Θ2|) of 5 deg. or less, and thus, offsetting the effects of thelateral grooves in inside and outside areas.

In the second embodiment an increase of the residue CF is prevented andan image of the pattern can be improved by constructing approximatelysymmetrical horizontal grooves in a reverse V shape in the right andleft outside areas SL and SR, or the right and left inside areas CL andCR, of which inclination angles (ΘsL, ΘsR), or (ΘcL, ΘcR) areapproximately equal, for example, the difference (|ΘsL-ΘsR|), or thedifference (|ΘcL|- |ΘcR|) is 5 deg. or less, and thus offsetting theeffects of the lateral grooves in the right and left inside areas, orthe right and left outside areas.

In addition to the above, in the first and second embodiments, bysetting the cord quantity NS at 15.0 mm² or less and the inclination ofthe belt cords at 21 deg. or more, the rigidity of the belt can bereduced and the riding comfort can be improved.

In order to obtain a compatibility of the steering stability with theone-side drifting performance of a car, the present inventors furthercontinued various studies. A conventional pneumatic radial tire forpassenger cars has rows of blocks formed by crossing the lateral groovesin a rib sectioned by plural circumferential grooves having a linear orzigzag shape which extend in a circumferential direction.

As a result of the studies, it was found that the contribution rating ofthese lateral grooves to the cornering performance of a tire is small,and therefore, they hardly affect the steering stability.

Moreover, it was also found from the examination that the residue CF isreduced by forming the outer lateral grooves constructed in the rightand left outside area S in a reverse direction to the outside belt cordswhich are the belt cords of the outermost belt ply and with aninclination of 0 to 40 deg. to the direction of tire's axis.

Prototypes of tires SA, SB and SC having outer lateral grooves Gs in theoutside area S in different directions were produced for thisexamination as shown in FIGS. 11(a) to (c). Outside belt cords 7a of theoutermost belt ply are shown by single-dotted broken lines in thefigures. In FIG. 11(a), the outer lateral grooves Gs are inclined in thesame direction as the outside belt cords 7a. In FIG. 11(b), they areformed in the direction of tire's axis. In FIG. 11(c), they are formedin the reverse direction. The results of measuring the residue CF insuch patterns as SA, SB and SC are shown in FIG. 13. In the patterns SA,SB and SC, the residue CFs are -14.4 kg, -7.8 kg and -3.6 kg,respectively. Thus, it is known the residue CF of the pattern SC inwhich the outer lateral grooves Cs are constructed in a differentdirection to the outside belt cords 7a is reduced.

FIG. 13 shows the residue CF in the cases of pattern CA, CB and CC. Inthe case of pattern CA, the inner lateral grooves Gc are inclined in thesame direction as the outside belt cords 7a in the inside area C, asshown in FIGS. 12(a), FIGS. 12(b) and (c) show the pattern CB in whichthey are inclined in the direction of tire's axis and the pattern CC inwhich they are inclined in the reverse direction, respectively. Theresidue CFs are -5.9 kg, -8.1 kg and -12.1 kg, respectively. Theabsolute value of the residue CF was reduced by inclining the innerlateral grooves in the same direction as the outside belt cords 7a.

Thus, it was found that the residue CF can be reduced and the one-sidedrifting performance of a car can be improved by inclining the lateralgrooves in the reverse direction to the outside belt cords 7a in theoutside area S and in the same direction in the inside area C.

Therefore, in the third embodiment outer horizontal grooves Gsconstructed in the outside area S are inclined in a different directionto the outside belt cords 7a and at 0 to 40 deg. to the direction oftire's axis.

In addition, in the third invention, lateral grooves are inclined in thesame direction as the outside belt cords 7a and at an angle of 40 deg.or less to the direction of tire's axis in the inside area C.

The following description relates to the fourth embodiment.

In the case that inner lateral grooves Gc having a larger inclinationare constructed in the inside area C, the cornering force upon turing ofa tire generated especially when the slip angle (α) is 1 deg. tends toreduced. Therefore, in some cases, it is not preferable to form innerhorizontal grooves Gc having a larger inclination angle.

Thus, it is required to impose the reducing effect of the residue CFcaused by the inside area C on the outside area S. For this purpose, thepresent inventors produced prototypes of tires having differentcircumferential pitches Ps in a direction of the tire's equator betweenthe outer lateral grooves Gs in the outside area S as shown in FIG. 26,and the results of measuring the residue CF are shown in FIG. 27.

From the results, it was found that the residue CF can be controlled by6 kg or less in absolute value by setting the circumferential pitch Psat 20 mm or less. Thus the cornering force generated when the slip angle(α) is 1 deg. can be larger and the steering stability upon turing canbe improved without relying upon the inner lateral grooves Gc of theinside area C.

Thus, the circumferential pitch Ps is set at 20 mm or less in the fourthinvention.

The next description relates to the fifth embodiment.

As priorly described, by constructing lateral grooves Gs in the outsidearea S in a different direction to the outside belt cords 7a, theresidue CF can be reduced, and the larger the inclination angle Cs is,the further the residue CF can be reduced.

However, it was found that in the case that the inclination (Θs) is setat a larger angle, the noise generated by the tread pattern becomeslarger on the other aspect. Therefore, when a noise characteristic isconsidered to be important, the inclination angle (Θs) of the outerhorizontal grooves Gs is limited.

Therefore, a controlling method of the residue CF was further studied.The residue CF was measured by changing the maximum length L in aright-angled direction to lateral grooves Gc of a block B formed by theinner lateral grooves Gc formed in a middle area M, dividing the treadpart shown in FIG. 32 into three equal areas. As known from the resultsshown in FIG. 34, it was found that the residue CF is reduced byreducing the maximum length L gradually. Furthermore, by setting themaximum length L at 10 mm or less, the absolute value of the residue CFcan be set at 5 kg or less.

Thus, the maximum length L of the block B in the middle area M was setat 10 mm or less in the fifth embodiment.

By this setting, the straight-forward driving stability can be improvedwithout affecting the steering stability or noise characteristic.

Moreover, in the third, fourth and fifth embodiments, this can bepreferably adopted in a tire having a belt which brings about a stronghooping effect and improves the steering stability, wherein the cordquantity NS is 18.0 mm² or more and the inclination of the belt cords tothe tire's equator is 18 deg. or less.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings, the embodiments of the present inventionare described in detail below, in which:

FIG. 1 is a sectional view showing one of the embodiments of the firstinvention,

FIG. 2 is a plan view showing one example of a tread pattern of theembodiment of FIG. 1,

FIGS. 3 to 6 are plan views showing other tread patterns, of theembodiment of FIG. 1, respectively,

FIG. 7 is a sectional view of a belt ply,

FIG. 8 is a sectional view showing an example of a belt cord,

FIG. 9 is a perspective view explaining the residue CF,

FIG. 10 is a diagram of the residue CF of FIG. 9,

FIGS. 11(a) to (c) and 12(a) to (c) are plan views showing patterns usedin the experiments, respectively,

FIG. 13 is a diagram showing the experimental results of the patternsshown in FIGS. 11(a) to (c) and FIGS. 12(a) to (c),

FIG. 14 is a plan view showing the tread pattern of one of theembodiments of the second invention,

FIGS. 15 to 18 are plan views showing other tread patterns,respectively, of the embodiment of FIG. 14,

FIG. 19 is a plan view showing the tread pattern of one of theembodiments of the third invention,

FIG. 20 is a plan view showing an example of another tread pattern ofthe embodiment of FIG. 20,

FIGS. 21 to 23 are plan views showing other tread patterns, respectivelyof the embodiment of FIG. 20,

FIG. 24 is a plan view showing the pattern of a comparison example,

FIG. 25 is a plan view showing the tread pattern of one of theembodiments of the fourth invention,

FIG. 26 is a plan view showing an example of another tread pattern ofthe embodiment of FIG. 26,

FIG. 27 is a diagram showing an example of the results of test,

FIGS. 28 and 29 are plan views showing other tread pattern,respectively, of the embodiment of FIG. 25,

FIG. 30 is a plan view showing the pattern of a comparative example,

FIG. 31 is a sectional view showing one of the embodiments of the fifthinvention,

FIG. 32 is a plan view showing an example of a tread pattern for theembodiment of FIG. 31,

FIG. 33 is a plan view showing an example of the other pattern,

FIG. 34 is a diagram showing an example of the test,

FIGS. 35 and 36 are plan views showing other patterns, respectively.

DETAILED DESCRIPTION OF THE INVENTION

In FIGS. 1 and 2, a pneumatic radial tire 1 of the first inventioncomprises a carcass 6 extending from a tread part 2 through a side-wallpart 3 to a bead part 4 and wrapped around a bead core 5, and a belt 7placed outside in the radial direction of the carcass 6 and inside thetread part 2.

The belt 7 comprises belt plies 7A and 7B of two inside and outsidelayers which are inclined in mutually reverse directions at aninclination angle (β) of 21 deg. or more to the tire's equator CO of thebelt cords. The belt ply cords 7a radially outer belt ply 7B in theembodiment are inclined in a right upper direction to the tire's equatorCO in FIG. 2. As belt cords, as shown for example in FIG. 8, twistedsteel filaments 7b of 2+7×0.22, 1×5×0.23 or 1×4×0.22, the last numeralbeing in mm² units, 31, for example, are used.

The cord quantity NS which is the product of a total cross-sectionalarea S (sq.mm) of one cord, that is, a sum of a cross-sectional area ofthe filament 7b of the belt cords and the number of cords N in adistance l of 10 cm in FIG. 7 is at 15.0 mm² or less, thereby therigidity of the belt 7 being reduced and the comfortable ridingperformance being improved.

In FIG. 2, the tread part 2 is virtually sectioned in the direction oftire's axis into a left inside area CL and a right inside area CR ofboth sides of the direction of the tire's equator CO, a left outsidearea SL and a right outside area SR that extend to the edges a of thetread part. In the embodiment, outer lateral grooves Gs comprising anouter groove part g1 with an inclination angle (Θ1) of 45 deg. or lessto the direction of tire's axis and on outer groove part g2 inclinedreversely to the outer groove part g1 at an angle (Θ2) and forming a Vshape with the outer groove part g1 are constructed in the right andleft outside area S. The difference of the inclination (Θ1) and (Θ2) ofthe outer groove part g1 and the inner groove part g2, (Θ1|-|Θ2|) is setat 5 deg. or less.

Thus, the outer lateral grooves Gs are about symmetrical in a directionof tire's axis, and the effect of the residue CF by inclinations isreduced. This also improves the appearance of a tire. In addition, inthe right and left inside area C, inner lateral grooves Gc inclining inright lower direction at an inclination angle (Θc) of 5 deg. or less tothe direction of tire's axis are provided.

(f an inclination angle (Θ1,Θ2) of the outer lateral grooves Gs exceeds45 deg., a pattern noise tends to be caused.

In this embodiment, circumferential grooves G1 and G2 are constructedcontinuously in the direction of tire's equator CO along the tire'sequator CO sectioning the right and left inside area C and at the middleposition between the tire's equator CO and the edge of the tread part 2,sectioning the right and left inside area C and the right and leftoutside area S. The circumferential grooves G (generally called thegrooves G1, G2) may be linear grooves or zigzag grooves.

Circumferential pitches Ps and Pc which are the distances between theouter and inner grooves Gs and Gc in the direction of tire's equator areboth set at 40 mm or less, preferably 20 mm or less.

FIG. 3 shows a case where the inner circumferential groove Gc in theinside area C is formed in a V shape. FIG. 4 shows an example in whichthe outer groove Gs is formed in a V shape and a groove Gc2 extending inthe direction of tire's axis and a V-shaped groove Gc1 are reciprocallyformed in the inside area C. A V-shaped horizontal groove may be formedin an inside area Gc or only in an outside area Gs.

FIGS. 2 to 4 show cases where main grooves G1 and G2 are constructedrespectively in the parts that section the inside area C and the outsidearea S, while in the case that two main grooves C2 and G2 are employed,as shown in FIG. 5, or in the case that four or more grooves G2A, andG2B are employed at a regular interval, as shown in FIG. 6, the insidearea C and the outside area S are considered to be sectioned by avirtual line F on the rib. At least one end of the inner lateral grooveGc must open to the circumferential or main grooves G.

One of the embodiments of the second invention is shown in FIG. 1 and14. Explanations about the similar or same composition as explained inthe foregoing first embodiment are omitted. The third, fourth and fifthembodiments hereinafter are treated in the same manner.

In the right and left outside area S, approximately symmetrical outerlateral grooves GsL and GsR (generally called outer lateral grooves Gs)are separately constructed with reverse inclinations of (ΘsL) and (ΘsR)of approximately 20 deg. to the direction of tire's axis in thedirection of tire's equator CO.

In addition, in the embodiment, approximately symmetrical inner lateralgrooves GcL and GcR (generally called inner lateral grooves Gc) areseparately constructed in the right and left inside area C as well withreverse inclinations of (ΘcL) and (ΘcR) of approximately 30 deg. to thedirection of tire's axis.

The differences of the inclination angles (|ΘsL|-|ΘsR|) and(|ΘcL|-|ΘcR|) should be set at 5 deg. or less.

Thus, the lateral grooves Gs and Gc are symmetrical in the direction oftire's equator CO and the effect of the residue CF by inclinations isreduced. The appearance is also improved.

In the inclinations (ΘsL), (ΘsR), (ΘcL) and (ΘcR) of the outer lateralgrooves Gs and Gc exceed 45 deg., a pattern noise tends to be generated.

Circumferential pitches Ps and Pc which are the distances between theouter and inner lateral grooves Cs and Gc in the direction of tire'sequator are both set at 40 mm or less, preferably at 20 mm or less.

FIG. 15 shows the other embodiment in which the inner lateral grooves Gcof the inside area C are formed in a reverse V shape and outer lateralgrooves Ga extend in the direction of the tire's axis FIG. 16 showsstill another embodiment in which the outer lateral grooves Gs areformed in a reverse V shape and inner lateral grooves Gc extend in thedirection of tire's axis.

FIGS. 14 to 16 show cases where main grooves G1 and G2 are constructedrespectively in the parts that section the inside area C and the outsidearea S, while in the case that two main grooves G2 and G2 are employed,as shown in FIG. 17, or in the case that four or more grooves G2A, andG2B are employed at a regular interval, as shown in FIG. 18, the insidearea C and the outside area S are considered to be sectioned by avirtual line F on the rib. At least one end of the inner circumferentialgroove Gc must open to the main grooves G.

One of the embodiments of the third embodiment is shown in FIG. 1 andFIG. 19.

In the right and left outside area S, outer lateral grooves Gs with aninclination angle (Θs) of 40 deg. or less to the direction of tire'saxis which is reverse to the inclination of the outside belt cords 7aare constructed at spacings in the direction of tire's equator CO. Inthe right and left inside area C, inner lateral grooves Gc with aninclination angle (Θc) of 40 deg. or less to the direction of tire'saxis inclined in the right upper direction same as the outside beltcords 7a are constructed. If the inclination exceeds the inclinationangle (Θs) of the outer lateral grooves Cs, a pattern noise tends to begenerated. In the case that the inclination (Θc) of the inner horizontalgrooves Gc exceeds 40 deg., the cornering force upon turning tends to bereduced, and the steering stability tends to be deteriorated.

Circumferential pitches Ps and Pc which are the distances between theouter and inner grooves Gs and Gc in the direction of tire's equator areset at 40 mm or less respectively, preferably at 20 mm or less.

It was described previously that the residue CF can be improved byconstructing outer grooves Gs reversely inclined to the outside beltcords 7a in the right and left outside area S, and inner grooves Gcinclined to the same direction in the right and left inside area C. Itis confirmed that the residue CF can be further reduced by setting thecircumferential pitches Pc and Ps preferably at 20 mm or less.

FIG. 20 shows the other embodiment where the inclination angle (Θs) ofthe outer grooves Gs is set at 0

FIGS. 19, 20 show cases where main grooves G1 and G2 are constructedrespectively in the parts that section the inside area C and the outsidearea S, while in the case that two main grooves G2 and G2 are employed,as shown in FIG. 21, or in the case that four or more grooves G2A, andG2A re employed at a regular interval, as shown in FIGS. 22 and 23, theinside area C and the outside area S are considered to be sectioned by avirtual line F on the rib. At least one end of the inner lateral grooveGc must open to the circumferential or main grooves G. As a belt cord,the same material as used in the first embodiment can be employed.However, by setting the cord quantity N S at 18.0 mm² or more, thehooping effect by the belt 7 is increased, and thus, the steeringstability is improved. It is the same in the fourth and fifthembodiments well.

One of the embodiments of the fourth invention is shown in FIGS. 1 and25.

In the right and left outside area S, outer lateral grooves Gs areconstructed in the direction of tire's equator with the main part Gs1that has a length exceeding 70% of the outside area S in the directionof tire's axis. The main part Gs1 is inclined at an angle (Θs) of 40deg. or less to the direction of tire's axis and reversely to theoutside belt cords 7a. In the main part Gs1, a short sub-part Gs2extending outward in the direction of tire's axis to the edges a of thetread part 2 is constructed. If the inclination angle (Θs) of the outerlateral grooves Gs exceeds 40 deg., a pattern noise tends to begenerated.

In the tread part 2, main grooves G2A and G2A are formed on the bothsides of the tire's equator CO, and other circumferential grooves G2Band G2B are also formed continuously in the direction of tire's equatorin the parts that section the inside area C and the outside area S.

The vertical grooves G (generally called the circumferential grooves G)may be linear grooves or zigzag grooves.

In the inside area C, an inner grooves GcA comprising an inner groovepart GcA1 extending inside from the main grooves G2A with the inner endsending near the equator of the tire CO and an outer groove part GcA2extending outside in the direction of tire's axis. Moreover, innergrooves GcB extending from the vertical groove G2B respectively to theinside direction of the tire are also constructed parallel in thedirection of tire's circumference. The inner grooves GcA and GcB areboth inclined in the same direction as the outside belt cords 7a. Bysetting the inclination angle (Θc) to the direction of the tire's axisat 35 deg. or less, the cornering force when the slip angle (α) is 1deg. is prevented from deteriorating, and the steering stability uponturning is prevented from reducing.

The circumferential pitch Pc of the inner groove Gc is set at 40 mm orless, preferably at 20 mm or less. In addition, by setting thecircumferential pitch Ps of the outer horizontal grooves Gs at 20 mm orless, the residue CF is reduced.

In the case that two main grooves G2 and G2 at a regular interval, asshown in FIG. 28, or four grooves G2A and G2B at a regular interval, asshown in FIG. 29, are employed, the inside area C and the outside area Sare sectioned by a virtual line F on the rib. At least one end of theinner grooves Gc must open to the vertical grooves G, and at least oneend of the outer grooves Gs must open at the edge a of the tread part orto the vertical grooves G.

One of the embodiments of the fifth invention is shown in FIG. 31 and32.

A tread part 2 is sectioned virtually in the direction of tire's axisinto a middle area M including the tire's equator CO and outward areasN, N extending to the edge a of the tread part outside the middle partM. In the outward area N, outer grooves Gs are constructed at spacingsin the direction of the tire's equator extending toward the direction oftire's axis with an inclination angle (Θn) of 0 deg. In the midde areaM, inner grooves Gc with an inclination angle (Θm) of 45 deg. or less tothe direction of tire's axis are constructed. In the embodiment, theinner grooves Gc are small grooves of 0.5 to 3 mm in width, and theinner grooves Gc from a crossing groove mutually inclined in reversedirections at approximately 40 deg. to the direction of tire's axis.Therefore, in the middle area M, multiple rhombic blocks B are formed inan oblique latticed shape. The maximum length L of the block B in aright-angled direction to the groove Gc is set at 10 mm or less.

In addition, in the embodiment, grooves G, G are continuouslyconstructed in the direction of tire's equator in the position tosection the middle area M and the outward area N into approximatelythree equal areas. The grooves G may be linear grooves or zigzaggrooves.

The circumferential pitch Ps which is the distances between the outerhorizontal grooves Gs in the direction of tire's equator is set at 40 mmor less, preferably at 20 mm or less.

By setting the length of the block B in the middle area M in aright-angled direction to the inner horizontal grooves Gc at 10 mm orless, even when the outer grooves Gs extending in the direction oftire's axis in the outward area N are constructed to control patternnoises, the residue CP can be reduced as mentioned before.

However, the outer grooves Gs are not so limited, and, as shown in FIG.33, they may be inclined in the direction of tire's axis at an angle(Θn) of such a range that does not increase pattern noises, for example,15 deg. or less, preferably 10 deg. or less, and more preferably 5 deg.or less.

FIGS. 32 and 33 show a case where vertical grooves G are respectivelyconstructed in the parts that section the middle area M and the outwardarea N, while three vertical grooves G1, G2 and G2 may be employed, asshown in FIG. 35, or four grooves G2A and G2B may be employed at aregular interval, as shown in FIG. 36. In these cases, the middle area Mand the outward area N are sectioned by a virtual line F on the rib,inner horizontal grooves Gc may be formed not in a latticed shape butalso as grooves parallel with the direction of the tire's axis orinclined and not mutually crossing, as shown in FIGS. 35 and 36.

EXAMPLES

A prototype of tire having a tire size of 175/70R13 was produced, andthe riding comfort and the residue CF were measured. As belt cords,steel cords of 1×4×0.22 in size were used. The belt was formed in twoplies. The test was performed by mounting the tire on a rim 5J×13,setting the internal pressure at 2.0 kg/sq.cm. loading 300 kg and usinga flat track machine prepared by MTS company, U.S.A. to measure theresidue CF. The residue CF is shown by a residue CF index setting theindex of the comparison example at 100 in Table 1 etc. The smaller theresidue CF index is, the more preferable the result is. In regard to theriding comfort, by mounting the tire on a 2,000 cc passenger car, afeeling test was conducted by a driver, and an evaluation was made bysetting the comparison example at 100 points. Higher scores show betterriding comfort.

A: In regard to the first embodiment of the invention, a prototype of atire as shown in Table 1, FIGS. 2 and 3 was produced. The results arealso shown in the Table 1.

B: In regard to the second embodiment of the invention, a prototype of atire as shown in Table 2. FIGS. 14, 15 and 16 was produced. The resultsare shown in the Table 2.

C: In regard to the third embodiment of the invention, a prototype of atire as shown in Table 3, FIGS. 19 and 20 was produced. The results arealso shown in the Table 3. As a comparative example, the other prototypehaving a pattern shown in FIG. 24 was also produced for the purpose ofcomparison.

D: In regard to the fourth embodiment of the invention, a prototype of atire as shown in Table 4 and FIG. 25 was produced. As a comparativeexample, the other prototype having a pattern shown in FIG. 30 was alsoproduced for the purpose of comparison, and the results are shown in theTable 4.

E: In regard to the fifth embodiment of the invention, a prototype of atire as shown in Table 5 and FIG. 32 was produced. The results are shownin the Table 5. The noise characteristic was also evaluated through afeeding test by a driver and shown in degrees of noise. Higher scoresmean more inferiority in noise characteristic.

Thus, the invention can improve the one-side drifting of a car withoutsacrificing the steering stability.

The invention being thus described, it will be obvious that the same maybe varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

                                      TABLE 1                                     __________________________________________________________________________             Em. 1-1                                                                            Em. 1-2                                                                            Em. 1-3                                                                            Em. 1-4                                                                            Co. 1-1                                                                            Co. 1-2                                                                            Co. 1-3                                                                            Co. 1-4                                                                            Co. 1-5                                                                           Co. 1-6                  Pattern  FIG.2                                                                              FIG.2                                                                              FIG.3                                                                              FIG.3                                                                              FIG.2                                                                              FIG.2                                                                              FIG.2                                                                              FIG.2                                                                              FIG.3                                                                             FIG.3                    __________________________________________________________________________    Belt cord                                                                     N × S                                                                            12.2 12.2 14.5 14.5 15.4 15.4 20.5 20.5 12.2                                                                              14.5                     Inclination angle                                                                      21   25   21   25   21   18   21   18   21  21                       Lateral groove                                                                in V shape                                                                    Location Outside                                                                            Outside                                                                            Inside                                                                             Inside                                                                             Outside                                                                            Outside                                                                            Outside                                                                            Outside                                                                            Inside                                                                            Inside                            area area area area area area area area area                                                                              area                     Inclination angle                                                                      35   35   35   35   30   30   20   20   30  20                       (θ1)                                                                    Inclination angle                                                                      35   40   35   40   40   40   40   40   40  40                       (θ2)                                                                    Inner lateral                                                                 groove                                                                        Inclination angle                                                                       2.5  2.5            2.5  2.5  2.5  2.5                              (θc)                                                                    Direction                                                                              Right                                                                              Right                                                                              --   --   Right                                                                              Right                                                                              Right                                                                              Right                                                                              --  --                                lower                                                                              lower          lower                                                                              lower                                                                              lower                                                                              lower                             Circumferential                                                                        18   18             18   18   18   18                                pitch Pc (mm)                                                                 Outer lateral                                                                 groove                                                                        Inclination angle  0    0                        10  10                       (Θs)                                                                    Direction                                                                              --   --   Lateral                                                                            Lateral                                                                            --   --   --   --   Right                                                                             Right                                                                     upper                                                                             upper                    Circumferential    18   15                       18  18                       pitch Ps (mm)                                                                 Riding comfort                                                                         107  110  102  104  100  97   95   93   106 102                      Residue CF index                                                                       80   35   90   50   100  130  110  140  100 120                      __________________________________________________________________________     Em.; Embodiment                                                               CO.; Comparative example                                                 

                                      TABLE 2                                     __________________________________________________________________________              Em.2-1                                                                            Em.2-2                                                                            Em.2-3                                                                            Em.2-4                                                                            Co.2-1                                                                            Co.2-2                                                                            Co.2-3                                      Pattern   FIG.14                                                                            FIG.14                                                                            FIG.15                                                                            FIG.16                                                                            FIG.14                                                                            FIG.15                                                                            FIG.16                                      __________________________________________________________________________    Belt cord                                                                     N × S                                                                             12.2                                                                              12.2                                                                              14.5                                                                              14.5                                                                              15.4                                                                              15.4                                                                              20.5                                        Inclination angle                                                                       21  25  21  25  21  18  21                                          Lateral groove GsL                                                            in left outside area                                                          Inclination angle                                                                       20  20  0   15  20  20  20                                          (θsL)                                                                   Direction Right                                                                             Right                                                                             --  Right                                                                             Right                                                                             Right                                                                             Right                                                 lower                                                                             lower   lower                                                                             lower                                                                             lower                                                                             lower                                       Circumferential                                                                         18  18  18  18  18  18  18                                          pitch Pc (mm)                                                                 Lateral groove GsR                                                            in right outside area                                                         Inclination angle                                                                       20  15  0   12  10  0   10                                          (θsR)                                                                   Direction Right                                                                             Right                                                                             --  Right                                                                             Right                                                                             --  Right                                                 upper                                                                             upper   upper                                                                             upper   upper                                       Circumferential                                                                         18  18  18  18  18  18  18                                          pitch Ps (mm)                                                                 Lateral groove GcL                                                            in left inside area                                                           Inclination angle                                                                       30  30  30  0   30  30  0                                           (θcL)                                                                   Direction Right                                                                             Right                                                                             Right                                                                             --  Right                                                                             Right                                                                             --                                                    upper                                                                             upper                                                                             upper   upper                                                                             upper                                           Circumferential                                                                         18  18  18  18  18  18  18                                          pitch Pc (mm)                                                                 Lateral groove GcR                                                            in right inside area                                                          Inclination angle                                                                       30  25  30  0   30  30  30                                          (θcR)                                                                   Direction Right                                                                             Right                                                                             Right                                                                             --  Right                                                                             Right                                                                             Right                                                 lower                                                                             lower                                                                             lower   lower                                                                             lower                                                                             lower                                       Circumferential                                                                         18  18  18  18  18  18  18                                          pitch Ps (mm)                                                                 Riding comfort                                                                          108 110 103 105 100 96  95                                          Residue CF index                                                                        73  24  78  35  100 160 130                                         __________________________________________________________________________     Em.; Embodiment                                                               CO.; Comparative example                                                 

                  TABLE 3                                                         ______________________________________                                                    Em. 3-1    Em. 3-2    Co. 3-1                                     Pattern     FIG.19     FIG.20     FIG.24                                      ______________________________________                                        Belt cord                                                                     Material    Steel      Steel      Steel                                       Number of plies                                                                            2          2          2                                          Cord        1 × 4 × 0.22                                                                 1 × 4 × 0.22                                                                 1 × 4 × 0.22                    N × S 20.5       20.5       20.5                                        Direction of outside                                                                      Right upper                                                                              Right upper                                                                              Right upper                                 belt cord                                                                     Inclination angle                                                                         16         16         16                                          Inner lateral                                                                 groove                                                                        Inclination angle                                                                         30         30         30                                          (θc)                                                                    Direction   Right upper                                                                              Right upper                                                                              Left upper                                  Circumferential                                                                           18         18         18                                          pitch Pc (mm)                                                                 Outer lateral                                                                 groove                                                                        Inclination angle                                                                         30         30         30                                          (θs)                                                                    Direction   Left upper Left upper Right upper                                 Circumferential                                                                           18         18         18                                          pitch Ps (mm)                                                                 Steering of outside                                                                       110        105        100                                         belt cord                                                                     Residue CF index                                                                           5         26         100                                         ______________________________________                                         Em.; Embodiment                                                               CO.; Comparative example                                                 

                                      TABLE 4                                     __________________________________________________________________________              Em. 4-1                                                                              Em. 4-2                                                                              Co. 4-1                                                                              Co. 4-2                                                                              Co. 4-3                                 Pattern   FIG.25 FIG.25 FIG.25 FIG.25 FIG.30                                  __________________________________________________________________________    Belt cord                                                                     Material  Steel  Steel  Steel  Steel  Steel                                   Number of plies                                                                          2      2      2      2      2                                      Cord      1 × 4 × 0.22                                                             1 × 4 × 0.22                                                             1 × 4 × 0.22                                                             1 × 4 × 0.22                                                             1 × 4 × 0.22                N × S                                                                             20.5   20.5   20.5   20.5   20.5                                    Direction of outside                                                                    Right upper                                                                          Right upper                                                                          Right upper                                                                          Right upper                                                                          Right upper                             belt cord                                                                     Inclination angle                                                                       18     18     18     18     18                                      Inner lateral                                                                 groove                                                                        Inclination angle                                                                       25     25     25     25     30                                      (θc)                                                                    Direction Right upper                                                                          Right upper                                                                          Right upper                                                                          Right upper                                                                          Left upper                              Circumferential                                                                         18     18     18     18     18                                      pitch Pc (mm)                                                                 Outer lateral                                                                 groove                                                                        Inclination angle                                                                       40     40     40     40     30                                      (θs)                                                                    Direction Left upper                                                                           Left upper                                                                           Left upper                                                                           Left upper                                                                           Right upper                             Circumferential                                                                         15     20     30     40     18                                      pitch Ps (mm)                                                                 Steering of stability                                                                   110    105    100    100    100                                     Residue CF index                                                                        24     26     39     49     100                                     __________________________________________________________________________     Em.; Embodiment                                                               CO.; Comparative example                                                 

                                      TABLE 5                                     __________________________________________________________________________                 Em. 5-1                                                                              Em. 5-2                                                                              Em. 5-3                                                                              Co. 5-1                                                                              Co. 5-2                                                                              Co. 5-3                                                                              Co. 5-4                Pattern      FIG.32 FIG.32 FIG.33 FIG.32 FIG.32 FIG.32 FIG.11(c)              __________________________________________________________________________    Belt cord                                                                     Material     Steel  Steel  Steel  Steel  Steel  Steel  Steel                  Number of plies                                                                             2      2      2      2      2      2      2                     Cord         1 × 4 × 0.22                                                             1 × 4 × 0.22                                                             1 × 4 × 0.22                                                             1 × 4 × 0.22                                                             1 × 4 × 0.22                                                             1 × 4 ×                                                                  1 × 4                                                                   × 0.22           N × S  20.5   20.5   20.5   20.5   20.5   20.5   20.5                   Direction of outside                                                                       Right upper                                                                          Right upper                                                                          Right upper                                                                          Right upper                                                                          Right upper                                                                          Right upper                                                                          Right upper            belt cord                                                                     Middle lateral                                                                groove                                                                        Inclination angle                                                                          30     30     30     30     30     30                            (θm)                                                                    Direction    Crossed                                                                              Crossed                                                                              Crossed                                                                              Crossed                                                                              Crossed                                                                              Crossed                                                                              --                     Maximum length L (mm)                                                                      5      10     10     15     20     30                            Outer lateral                                                                 groove                                                                        Inclination angle                                                                          Lateral                                                                              Lateral                                                                                8    Lateral                                                                              Lateral                                                                              Lateral                                                                              35 (θs)          (θn)                                                                    Direction    --     --     Right lower                                                                          --     --     --     Right lower            Circumferential                                                                            18     18     18     18     18     10     18                     pitch Ps (mm)                                                                 Steering stability                                                                         105    105    105    100    95     95     105                    Residue CF index                                                                           70     30     80     100    135    170    70                     Noise characteristic                                                                       90     95     105    100    105    110    115                    __________________________________________________________________________     Em.; Embodiment                                                               CO.; Comparative example                                                 

What we claim is:
 1. A pneumatic radial tire comprisinga pair of beadportions, a bead core disposed in each of said bead portions, a carcassextending between the pair of bead portions and turned up around each ofthe bead cores, a belt comprising at least two plies of steel cordsdisposed radially outside the carcass, said steel belt cords in each ofsaid at least two belt plies laid at an inclination of 18 degrees orless with respect to the tire equator, one of said at least two pliesbeing the radially outermost ply, each of said at least two belt plieshaving a cord quantity NS of 18.0 mm² or more, wherein the cord quantityNS is defined as the total cross sectional area of said steel belt cordsS (mm²) per 10 centimeters width of belt ply, a tread portion disposedradially outside the carcass and having a pair of tread edges, saidtread portion including only four straight circumferential grooves madeup of a pair of axially outer circumferential grooves and a pair ofaxially inner circumferential grooves, said tread portion being dividedin the axial direction of the tire into four circumferential regionshaving substantially equal axial widths made up of a pair of axiallyinner circumferential regions, one of said inner circumferential regionsextending axially outward from each side of the tire equator, and a pairof axially outer circumferential regions, one of said outercircumferential regions extending axially inward from each said treadedge, one of said pair of axially outer circumferential grooves disposedaxially between the axially inner and outer circumferential regions, andone of said pair of axially inner circumferential grooves disposed ineach said axially inner circumferential regions, said tread portionbeing provided with a plurality of first lateral grooves, a plurality ofsecond lateral grooves and a plurality of third lateral grooves, saidfirst lateral grooves being disposed in each of said outercircumferential regions, each of said first lateral grooves having afirst part and a second part, said first part having an axially outerend and a length exceeding 70% of the axial width of each of said outercircumferential regions and being inclined at a first inclination angleof 40 degrees or less with respect to the axial direction of the tire,said first inclination angle being reverse to the inclination of thecords of the outermost ply, said second part extending between saidouter end to said tread edge, said second lateral grooves being disposedin each of said inner circumferential regions, each of said secondlateral grooves being divided by intersection with one of said axiallyinner circumferential grooves into a primary part and a secondary part,each said primary part and each said secondary part having a blind end,said blind end of said secondary part being disposed axially near thetire equator and said blind end of said primary part being disposedaxially apart from the tire equator, said second lateral grooves beinginclined at a second inclination angle of 35 degrees or less withrespect to the axial direction of the tire, said second inclinationangle inclining along the inclination of said cords of the outermost plyand said second inclination angle being reverse to said firstinclination angle, and said third lateral grooves being disposed to eachof said inner circumferential regions, each of said third lateralgrooves having an axially inward blind end and an axially outward endwhich opens to one of said axially outer circumferential grooves andbeing inclined at a third inclination angle of 35 degrees or less withrespect to the axial direction of the tire, said third inclination angleinclining along the inclination of said cords of the outermost ply. 2.The tire according to claim 1, wherein a first circumferential pitchbetween each adjacent pair of said first lateral grooves issubstantially 20 mm or less, a second circumferential pitch between eachadjacent pair of said second lateral grooves is substantially 20 mm orless, and a third circumferential pitch between each adjacent pair ofsaid third lateral grooves is substantially 20 mm or less.